1. Field of the Invention
The present invention relates to an integrated circuit, and more particularly to a semiconductor device.
2. Description of Related Art
MOS devices are widely used semiconductor devices. As the sizes of devices are gradually reduced, channel lengths are reduced, which accelerates the operation speed of MOS transistors, and meanwhile aggravates a short channel effect. Based on a formula of Electric field=Voltage/Length, if an applied voltage remains unchanged, and the channel length of a transistor is reduced, the energy of electrons in the channel will be increased under the acceleration of the electric field, thus resulting in more frequent electrical breakdown phenomena. The increase of intensity of the electric field may enhance the energy of electrons in the channel, which also results in the electrical breakdown.
Generally speaking, the breakdown voltage of a high voltage device happens earlier since the potential crowding phenomenon happens earlier on a surface of a side close to the drain region, such that it is difficult to raise the breakdown voltage. The conventional high voltage device increases the distance between a source/drain region and a gate region so as to reduce a transverse electric field in the channel by the use of an isolation structure. Moreover, the field implant region formed below the isolation structure may achieve a channel stop function, thereby improving the effect of breakdown voltage of the device.
FIG. 1 is a schematic cross-sectional view of a MOS device. Referring to FIG. 1, in order to raise the breakdown voltage of the device, a field implant region 112 is usually formed below the isolation structure 102. However, since the formed field implant region 112 usually not only is located below the isolation structure 102, but also extends upwards to be neighboring to the channel region 104. When the MOS device is applied in a high voltage, the voltage difference between the substrate 100 and the gate 124 is large. When a certain voltage difference is reached, the channel region 104 is inversed. The formed inversion layer and the field implant region 112 may form a PN junction, thus causing a severe leakage current.